ITC, Sensitivity and Overheads

The GNIRS Integration
Time Calculator (for spectroscopy only) can be used to determine limiting magnitudes, exposure times, S/N ratios, background levels, etc. for a wide range of source properties, observing conditions and GNIRS configurations. When using the ITC please note the caveats and guidelines at the top of the ITC web page.

A table of spectroscopic sensitivity estimates is also available. Note that it applies to specific wavelengths at which the atmosphere is nearly transparent, and should not be taken as indicative of the sensitivity at other wavelengths in its band. The ITC must be used to determine sensitivities at other wavelengths within a band.

It is important to note that very high values of S/N predicted by the ITC are unrealistic; i.e., sensitivity and S/N are only interchangeable for faint sources / low S/N. To achieve very high S/N requires stability of the instrument against small wavelength shifts and shifts in the optical path, stability over time of the pixel-to-pixel response, the accuracy of removal of telluric lines (and thus to some extent the stability of atmospheric conditions), and other factors, none of which are included in the ITC. In practice ground-based infrared observations rarely achieve signal-to-noise ratios greater than several hundred. Scientists who wish to obtain values of signal-to-noise ratio considerably higher than 100 should consult with GNIRS scientists prior to proposal submission.

The GNIRS ITC may be used to determine if an observation will saturate the array. Further information is given here.